1. Field of the Invention
The present invention relates to a laser amplifier and oscillator using a medium doped with a rare earth element as a gain medium and using a semiconductor laser as an excitation light source, and a method and apparatus for laser amplification using the same.
2. Description of the Related Art
A laser apparatus using a crystal or glass medium doped with a rare earth element as an active medium, that is, a laser oscillator and amplifier or the like is widely applied to information communication industries and mechanical engineering fields. An optical fiber amplifier in a large yield solid laser for metal processing and an optical fiber communication system is a typical example of its application.
The efficiency, size, service life, and mechanical stability of the laser oscillator and amplifier are mainly determined depending on an excitation light source. A semiconductor laser used as an excitation light source is superior to a solid laser and a fiber laser in these respects. Therefore, considering equipment applicability, it is preferred to employ a system (LD excitation system) using a semiconductor laser (laser diode: LD) as an excitation light source. A solid laser and a fiber laser used as an excitation light source have many more disadvantages than the semiconductor laser in various respects. In particularly, apart from the fact that the solid and fiber lasers are disadvantageous as compared with the LD in view of efficiency, size, mechanical stability, and service life, these lasers are disadvantageous in that a small number of passive optical parts in a bandwidth of 1.05 xcexcm in wavelength is provided. For example, an optical isolator with its low loss and high isolation is not practically used.
In such a laser apparatus, energy is supplied to rare earth ion by means of light excitation. Thus, in order to ensure operation with high efficiency, it is particularly important to select a wavelength of an excitation light source. However, some kinds of rare earth ions do not match well a wavelength of a semiconductor laser in optimal excitation wavelength, i.e., ion absorption wavelength bandwidth, making it difficult to ensure LD excitation caused by a semiconductor laser with its single wavelength. Thus, it may be required to use a light source other than LD. In particular, in the case where an ion forming a self-termination system whose laser low level service life is longer than laser high level service life is used as a gain medium, as shown in the following example, the wavelength of excitation light is limited more significantly, thus making it more difficult to ensure LD excitation.
As an example, a case of a Tm (thulium) ion in a fluorine glass will be described here. When a rare earth element such as Tm is doped in a medium such as fluorine glass, the element is ionized in the medium to form a Tm ion. FIG. 1 is an energy level chart showing a conventional method of exciting a thulium fiber amplifier. In addition, the wavelength of the conventional excitation light is explicitly shown in the figure. FIG. 2 is a graph illustrating an ASE (amplified spontaneous emission) spectrum when a transition shown in FIG. 1 is generated. As shown in FIG. 1, in a fiber amplifier in which a Tm ion is doped in a core, 1.04 to 1.07 xcexcm (hereinafter, referred to a bandwidth of 1.05 xcexcm) is used as an excitation wavelength, whereby light amplification of 1.47 xcexcm bandwidth in wavelength (transition from 3F4 to 3H4) can be achieved. In the figure, the light amplification is explicitly shown as a transition xe2x80x9caxe2x80x9d. In addition, at this time, an ASE spectrum as shown in FIG. 2 can be obtained. In more detail, this fact is disclosed in IEEE Journal of Quantum Electronics, Vol. 31, page 1880, 1995; Japanese Patent Application No. 11-156745; and Optics Letters, Vol. 24, page 1684, 1999.
In such a fiber amplifier, as shown in FIG. 1, an excitation photon with its 1.05 xcexcm bandwidth causes Tm ion base level absorption (transition from 3H6 to 3H5), and further causes non-radiation transition (not shown) and excitation state absorption (transition from 3H4 to 3F2 or transition from 3F4 to 1G4). Then, an inversion distribution is formed between 3F4-3H4 levels due to two-stage transition. The reason why this technique is efficient is that Tm ion base level absorption spectrum and excitation level absorption spectrum are superimposed in wavelength of 1.05 xcexcm, thus making it possible to ensure excitation with single excitation light of 1.05 xcexcm in wavelength.
However, in the above-mentioned Tm doped fiber amplifier, it is difficult to ensure excitation of 1.05 xcexcm bandwidth with a semiconductor laser. This is because, although laser light oscillation of 1.05 xcexcm bandwidth in a semiconductor laser is reported in some research papers, an apparatus capable of achieving a practical yield power level, for example, a transverse single mode yield of about 500 mW, does not exist in research level and commercially available level. For example, as disclosed in Applied Physics Letters, Vol. 69, page 248, 1996, the current semiconductor laser yield of 1.06 xcexcm in wavelength is about 200 mW. The current semiconductor laser yield disclosed in the paper is too small to ensure excitation in the above-mentioned Tm doped fiber amplifier. And the yield of commercially available semiconductor laser is smaller than that.
For such reasons, in a conventional Tm-doped fiber amplifier, as an excitation light source of 1.05 xcexcm bandwidth, for example, there is used LD excited solid lasers such as Nd: YAG, Nd: YLF, Yb: YAG; or LD excited fiber laser such as Yb doped fiber lasers, for example.
On the other hand, although excitation light source other than 1.05 xcexcm bandwidth, for example, excitation of 0.79 xcexcm bandwidth that directly excites 3F4 level, for example, (wavelength of 0.77 to 0.80 xcexcm and transition xe2x80x9cbxe2x80x9d in FIG. 1) and 0.67 xcexcm bandwidth that excites 3F2 level (a wavelength of 0.64 to 0.68 xcexcm and transition xe2x80x9ccxe2x80x9d in FIG. 1) can perform LD excitation, for example, as disclosed in Electronics Letters, Vol. 25, page 1660, 1989, the ion number density of laser low level (3H4) increases, and an inversion distribution cannot be maintained in a constant state, making it impossible to ensure operation with high efficiency. This is because the laser low level service life is about 10 msec in Tm ion, which is longer than the laser high level service life (3F4 service life is 1.3 msec). Such a system is called self-termination system, which is observed in rare earth elements Er (Erbium) and Ho (Holmium) or the like other than Tm.
In a laser amplifier and oscillator using a rare earth element forming its self-termination system, in order to ensure operation with high efficiency, it is essentially required to provide excitation light serving to excite ion from a base level to laser low level or an energy level above the laser low level; and excitation light serving to excite ion from laser low level to laser high level, and to form an inversion distribution. As described above, the excitation light of 1.05 xcexcm bandwidth in Tm can play these two roles at the same time, but LD excitation is impossible.
As shown in the above example, in the case where a medium in which an ion forming its self-termination system is doped is employed as a gain medium, an excitation wavelength is limited, thus making it difficult to ensure LD excitation.
It is an object of the present invention to provide a laser amplifier, a method and an apparatus for laser amplification, and a laser oscillator, which uses a medium doped with a rare earth element forming its self-termination system transition, in which semiconductor laser excitation is enabled and high efficiency, miniaturization, extended service life, and highly stable operation can be ensured at the same time.
A laser amplifier according to the present invention is directed to a laser amplifier using a medium doped with a rare earth element as a gain medium, and employing inductive discharge transition between two energy levels higher than a base level among energy levels of rare earth ion in the medium. The inductive discharge transition forms a self-termination system transition whose laser low level service life of the two energy levels is longer than laser high level service life of the two energy levels. The laser amplifier comprises: a first excitation light source for exciting ion from a base level to the laser low level or an energy level upper than the laser low level; and a second excitation light source having a wavelength different from that of the first excitation light source and for exciting ion from the laser low level to the laser high level, wherein at least one of the first and second excitation light sources is composed of a semiconductor laser.
In addition, the laser amplifier of the present invention can use fluoro zircon ate glass as the medium doped with a rare earth element.
Further, in the laser amplifier of the present invention, the rare earth ion may be thulium ion (Tm3+) and the first excitation light source may have any one of three wavelength ranges from 1.53 xcexcm to 1.90 xcexcm, 0.77 xcexcm to 0.80 xcexcm and 0.64 xcexcm to 0.68 xcexcm and the second excitation light source may have a wavelength range of 1.35 xcexcm to 1.46 xcexcm. In addition, the laser amplifier of the present invention preferably has an optical fiber shaped medium.
A laser amplification method according to the present invention arranges a plurality of laser amplifiers including the above-mentioned laser amplifier in series or parallel, thereby broadening a gain in bandwidth.
A laser amplification apparatus according to the present invention has a plurality of laser amplifiers including the above-mentioned laser amplifier arranged in series or parallel.
A laser oscillator according to the present invention is directed to a laser oscillator using a medium doped with a rare earth element as a gain medium, and employing inductive discharge transition between two energy levels higher than a base level among energy levels of rare earth ion in the medium. The inductive discharge transition forms a self-termination system transition whose laser low level service life of the two energy levels is longer than laser high level service life of the two energy levels. The laser oscillator comprises: a first excitation light source for exciting ion from a base level to the laser low level or an energy level upper than the laser low level; and a second excitation light source having a wavelength different from that of the first excitation light source and for exciting ion from the laser low level to the laser high level, wherein at least one of the first and second excitation light sources is composed of a semiconductor laser.
The laser oscillator of the present invention can use fluoro zircon ate glass as the medium doped with a rare earth element.
Further, in the laser oscillator of the present invention, the rare earth ion may be thulium ion (Tm3+) and the first excitation light source may have any one of three wavelength ranges from 1.53 xcexcm to 1.90 xcexcm, 0.77 xcexcm to 0.80 xcexcm and 0.64 xcexcm to 0.68 xcexcm and the second excitation light source may have a wavelength range of 1.35 xcexcm to 1.46 xcexcm.
In the present invention, in a laser amplifier and oscillator in which a rare earth element forming a self-termination system transition is doped, a semiconductor laser light source having two properly selected wavelengths is used as an excitation light source.
First, two excitation light actions in the present invention will be described. A first excitation light excites ion from a base level to a laser low level or an energy level above the laser low level. This first excitation light serves to efficiently excite ion to an energy level group associated with inductive discharge transition, i.e., to laser high level and laser low level. At this time, the first excitation light may not necessarily excite ion up to an energy level above the laser high level. In addition, in the case where irradiation is performed with only the first excitation light, since the energy level is a self-termination system, the ion number density of the laser low level increases, and a constant inversion distribution is not formed.
Next, ion is excited from the laser low level to the laser high level by means of a second excitation light. In this manner, an inversion distribution is formed between desired energy levels, and laser amplification operation in its inductive discharge transition is achieved.
As the first excitation light, there may be selected a wavelength that matches base level absorption transition to a laser low level or an energy level above the laser low level. As compared with a case of single wavelength excitation, much more selections can be made, and semiconductor laser excitation is also possible. As the second excitation light, there may be selected a wavelength that corresponds to an energy gap between the laser high level and the laser low level. This can be accomplished by selecting a light source with its wavelength slightly shorter (about 0.02 to 0.10 xcexcm) than the inductive discharge transition wavelength of interest. The second excitation light can be achieved by the semiconductor laser as long as the inductive discharge transition can be achieved by the semiconductor laser.
In addition, in an excitation arrangement of the present invention, in the case where a wavelength of the first excitation light is set so as to correspond to an energy gap between the base level and the laser low level, energy conversion efficiency, i.e., slope efficiency becomes maximal. The reason is stated as follows. In general, an energy loss component due to non-radiation transition becomes a main cause of lowering energy conversion efficiency. However, in the case where the wavelength of excitation light is set as described above, the energy loss component lost by non-radiation transition is very small. In simple estimation that ignores the width of each energy level, in the case of 1.05 xcexcm excitation in a Tm doped fiber amplifier, the theoretical maximum value xcex7s of the slope efficiency of the fiber amplifier is 1.05/1.46/2=36%. However, in the case where the wavelength of the first excitation light is defined as 1.56 xcexcm, and the wavelength of the second excitation light is defined as 1.46 xcexcm, the slope efficiency reaches 50%. In the case of the laser oscillator, this tendency is more significant. With respect to the theoretical slope efficiency in excitation power that is about 5 times of threshold, in the case of 1.05 xcexcm excitation, xcex7s=73%. However, in the case of 1.42 xcexcm excitation+1.56 xcexcm excitation, xcex7s=97%. Therefore, in the present invention, there can be provided a laser apparatus with its high operation efficiency.
Now, a gain medium will be described below. Hereinafter, the present invention will be described by exemplifying Tm as a rare earth element doped in a gain medium. The rare earth elements which can be used in the present invention can employ inductive discharge transition between the laser high level and the laser low level with two energy levels being higher than the base level. It is sufficient if these two levels are made of self-termination system transition, and at least one of the first and second excitation lights matching the existing semiconductor laser wavelength can be selected in consideration of energy level possessed by ion, without being limited to Tm.
Although it is sufficient if a medium doped with a rare earth element is employed as a medium of a general solid laser or fiber laser, such a medium is generally made of a glass. For example, there can be exemplified quartz, phosphate glass, borate glass, germanium glass, tellurite glass or fluoro zircon ate glass and the like. Among them, fluoro zircon ate glass is preferable because the glass is the lowest in phonon energy, non-radiation transition does not occur even in transition with a small energy difference, and energy can be taken out as light by radiation transition. In addition, when a medium is fiber-formed, a gain can be obtained by its length, and thus, it is desirable that the medium is fiber-shaped.
In this manner, in the laser amplifier according to the present invention, there are provided two light sources for oscillating first and second excitation lights, thereby enabling semiconductor laser excitation. Therefore, there can be eliminated problems caused by employing a light source such as a solid laser or fiber laser as an excitation light source, and operation with high efficiency can be achieved. In addition, the present invention can be configured as a laser oscillator by adding a general resonator structure in addition to a laser amplifier. Further, a plurality of the laser amplifiers are connected in series or parallel, whereby a laser amplification apparatus with its broadened gain in bandwidth can be configured.
As has been described above, according to the present invention, the laser amplifier, laser amplification apparatus, and laser oscillator in which ion forming a self-termination system is doped, enable semiconductor laser excitation, and there can be obtained a laser amplifier, laser amplification apparatus, and laser oscillator with high efficiency, small size, long service life, and high stability. Further, according to the present invention, there can be provided a laser amplifier having its long-wavelength-shifted gain peak wavelength and a laser oscillator having its long-wavelength-shifted oscillation wavelength. Therefore, a fiber amplifier free of a gain peak shift and a fiber amplifier with its shifted gain peak are connected to each other in series or parallel, whereby there can be provided a laser amplification apparatus having its broad amplification wavelength bandwidth. This laser amplification apparatus can be used for multiple wavelength communication that can cope with a large capacity.